A CVT (Continuously Variable Transmission) automatic transmission of the type described at the start is generally known from practice. Such an automatic transmission, also known as a belt transmission, commonly comprises a variator for the continuous adjustment of a transmission ratio, which comprises a first conical pulley set, known as the primary pulley set, on a drive input shaft or primary shaft, and a second conical pulley set, known as the secondary pulley set, on a drive output shaft or secondary shaft. Each pair of conical pulleys consists of a first pulley fixed in the axial direction and a second pulley that can move in the axial direction, which is called the primary pulley or the secondary pulley depending on whether it is associated with the primary or the secondary pulley set. Between these pulley pairs there runs a torque transmission element which, for example, can be a thrust jointed band.
To displace the primary or the secondary pulley, these are acted upon by a pressure medium from a pressure source, and in each case the pressure acting on the pulley sets or on the regulating volume of the axially movable pulleys is adjusted with the aid of an electronic transmission control and an electro-hydraulic pressure delivery device.
From industrial practice hydraulic systems for the control of a CVT automatic transmission are known, in which the pressure on the primary pulley set is limited to the maximum pressure exerted on the secondary pulley set, which as a rule determines the contact pressure. The value controlling the pressure on the secondary pulley set then at the same time controls the main pressure to be supplied by the oil pump.
For reliable transmission at the primary and secondary pulley sets, there must be a guaranteed pressure at every operating point in both adjustment pulleys. To move the variator in the overdrive direction, i.e. when “changing up”, the primary pulley set must be acted upon by a certain pressure. In this, the pressure level influences the shift speed of the variator and is therefore determined by the operating point characteristics of the driving strategy.
However, when the secondary valve that controls the secondary pulley set determines the main pressure, the pressure on the primary pulley set can only be lower or the same size. This limitation, namely that the primary pulley set pressure can be made at most equal to the secondary pulley set pressure, places a restriction on the pulley area ratio of the primary and secondary pulleys, such that the said ratio must correspond at least to the maximum supporting force ratio of the variator. In this, high contact forces on the primary pulley can only be achieved if the primary pulley has a substantially larger diameter than the secondary pulley, which is unfavorable in relation to the restricted structural space situation.
DE 199 06 558 shows a hydraulic system for the control of a CVT automatic transmission, which differs from the above in comprising a separate main pressure valve control system. By virtue of a main pressure control system with a separate main pressure valve and all the associated pressure regulator chains and a pressure sensor, the above described limitation of the pressure on the primary pulley set to the maximum pressure of the secondary pulley set can be overcome.
Disadvantages of this solution, however, are the cost entailed for the pressure regulator and perhaps pressure sensor, and the dependence on pressure regulator and pressure sensor tolerances of the supply pressure offset adjusted in accordance with the software. The tolerances of the pressure regulators and sensors used act additively on the primary and secondary sides, so that the minimum possible offset is not always achieved.
The purpose of the present invention is, therefore, to provide a hydraulic system of a CVT transmission of a motor vehicle, with which the pressure of the variator can be controlled reliably with the simplest possible means and with which a geometrically favorable pulley ratio is possible.
According to the invention, this objective is achieved with a hydraulic system having the characteristics of claim 1.